Precipitation effects on soil erosion.
The R factor in the USLE embodies effects on soil erosion by precipitation events. For rain induced soil erosion events are relevant, which have an intensity above 1,27 to 12,5 mm/h within a 30 minute time interval. Often 10 mm/h is chosen as threshold. Heavy rain of this magnitude possesses enough kinetic energy for mobilizing soil material when hitting the ground. If the intensity succeeds the rate of infiltration into the soil body, surface runoff will cause washdown of soil particles. In areas with known erosion events following snow melt, the precipitation values should be increased intentionally (e. g. during the months december until march, depending on location) in order to account for these melt water effects.
For some regions regression equations exist which enable the estimation of the R factor based on the longterm mean precipitation. Here the equation R = 0,1039*P-7,31 (Sauerborn 1994) has been used. Information on longterm mean precipitation have been taken from DWD raster data. By using the Grid Calculator in SAGA GIS R factor was calculated for each 100x100m raster cell. Other approaches solely use mean summer precipitation or explicitly incorporate extreme rain fall events.
R factor values are given in kJ/m²*mm/h. As longterm mean precipitation does not vary vastly within smaller areas, it is not surprising that R factor values are fairly equal. Values are increasing towards the south-western end in the map above due to higher elevation (see article with elevation map). Areas with high yearly precipitation volumes or a remarkable number of intense rain fall events could hold R factors of 1000 and higher.
SAUERBORN, P. (1994): Erosivität der Niederschläge in Deutschland – Ein Beitrag zur quantitativen Prognose der Bodenerosion durch Wasser in Mitteleuropa. Bonner Bodenkundliche Abhandlungen, Band 13. Bonn.